1. Field of the Invention
The present invention is generally related to the metallization used in vias in glass-ceramic electronic structures and, more particularly, to a low-cost ternary composite for use in such glass-ceramic structures.
2. Description of the Prior Art
FIG. 1 shows the general design of a prior art input/output (I/O) pad for a glass-ceramic electronic structure. A capture pad 10 is evaporated on top of a glass-ceramic structure 12 over metal filled vias 14 and 16. The capture pad 10 typically comprises a multilayered structure of titanium, copper and chromium (e.g., 0.2 .mu.m Ti/30 .mu.m Cu/0.1 .mu.m Cr). The metal filled vias 14 and 16 typically comprise a good metallic conductor such as copper. The copper via is capable of forming a tight bond with the capture pad 10; however, there is generally poor adhesion between the copper via and the glass-ceramic structure 12 which leads to the formation of voids and channels that need to be filled to prevent liquids and gases encountered during and after processing from entering the vias 14 and 16 and causing damage. To provide this protective seal, a polyimide cushion 22 is applied over the glass-ceramic structure 12 and capture pad 10. A small opening is then made in the polyimide cushion 22 above the vias 14 and 16, and a thin film bottom surface metallurgy (BSM) pad 24 is bonded to the capture pad 10 through the opening. The BSM pad 24 is typically a layered structure of chromium, copper, titanium and gold (e.g., 0.1 .mu.m Cr/6.0 .mu.m Cu/1.0 .mu.m Ti/1.0 .mu.m Au). A pin 26 is connected directly to the BSM pad 24 using Au/Sn eutectic braze 28. For example, a Au/Sn eutectic having 20 wt % Sn can be brazed to the BSM pad 24 at 380.degree.-400.degree. C. In addition to providing a protective polyimide barrier for the vias 14 and 16, the structure shown in FIG. 1 has been used instead of bonding the pin 26 directly to the capture pad 10 because direct bonding tends to transfer stress from the pin 26/braze 28/pad 10 structure to the glass-ceramic 12 and may cause cracking. Any viable solution to pin 26 bonding must minimize the transfer of stress to the glass-ceramic 12 to prevent cracking.
The prior art structure shown in FIG. 1 has several disadvantages. First, the chromium layer of the BSM pad 24 has poor adhesion to the polyimide cushion 22. Moreover, the polyimide cushion 22 tends to degrade in humid environments which may be encountered in the field and this degradation will further exacerbate the poor adhesion problem of the BSM pad 24. Because the area where the BSM pad 24 and the capture pad 10 bond together is relatively small compared to the total areas of the pads, any loss of adhesion with the polyimide cushion 22 will drastically reduce the strength of the pin 26 joint. Second, producing the pin 26 connection is very expensive and involves the evaporation of seven metal layers for the capture pad 10 and BSM pad 24 (thin film technology), and the application of a polyimide layer for the polyimide cushion 22, as well as the formation of a via in the polyimide cushion 22. Third, if a failure occurs anywhere in the film structure of the joint, the joint cannot be reworked because of the manufacturing process employed. Thin film structures cannot be easily reworked due to process complexity and cost.
Metal-glass conductors are well known in the art. Milkovich et al. U.S. Pat. No. 3,537,892 discloses metallized conductors comprised of gold, platinum, and palladium which are mixed with vitreous frit. The powdered metals and glass are sifted, mixed together, combined with a vehicle to form a paste, and the paste is printed on a substrate followed by drying and firing. Milkovich et al. anticipate using a large weight percentage of gold in the mixture which makes the conductors very expensive (e.g., 60-75% gold, 10-25% platinum, and 5-30% palladium). Moreover, Milkovich et al. is directed to producing conductive lines on a substrate, as opposed to metallized vias in a substrate. Morse U.S. Pat. No. 3,934,336 discloses the use of a sintered gold-glass conductive material in a ceramic substrate of an electronic package. As pointed out by Morse, the glass binder provides for good adhesion with the ceramic substrate. Nishigaki et al. U.S. Pat. No. 4,650,923 describes a ceramic article which utilizes a silver-palladium mixture in the vias, and Tsunashima U.S. Pat. No. 4,323,593 discloses the formation of noble (gold, silver, or platinum) metal-glass vias in printed circuit boards (PCBs).